Generally, when using a paging receiver, if a word synchronization code is not detected from a received RF paging signal due to a weak field strength or fading phenomenon, etc., the paging receiver cannot receive self-calling data, i.e., data containing the unique address code of the paging receiver.
Ordinary battery saving circuits are used to minimize power consumption in a RF paging receiver by periodically supplying power instead of continuously. The battery saving function of these battery saving circuits operates in dependence upon detection of an RF carrier. If the RF carrier is found, the battery power is successively supplied to a RF receiving unit of the paging receiver for a predetermined time. This conventional method however has a low battery saving efficiency because all individual receivers within a radio paging system must be activated whenever the RF carrier is found, regardless of which individual paging receiver is intended to be selectively reached. A more advanced battery saving method is to individually receive a RF paging signal in response to a RF carrier in a predetermined call signaling format, but it still suffers similar low battery saving efficiency. The call signaling format widely used exclusively for most paging receivers in the world today is the POCSAG (Post Office Code Standardization Advisory Group) code. POCSAG code is specified in CCIR (International Consultant Commmittee for Radio) recommendation No. 584 to include a preamble code followed by a plurality of batches which respectively indicate call numbers. Each batch begins with a word synchronization code adapted to achieve frame synchronization, followed by code words representative of address and message code words.
In the calling procedure of a conventional paging receiver, if a word synchronization code is not detected, subsequent address code words representative of address and message code words are also not detected. That is, under the operation of a battery saving function, the paging receiver searches for a preamble code (i.e., a first predetermined code) from a received RF paging signal. If the preamble code is found, the paging receiver searches for a word synchronization code (i.e., a second predetermined code) of a first batch. If the word synchronization code is found, subsequent specific address codes representative of a calling number and messages are detected. Thereafter, the paging receiver proceeds to search for a word synchronization code of a next batch. If the word synchronization code of the next batch is not found due to occurrence of errors in the second predetermined code at the next batch, any subsequent address code word is also not detected. This type of a paging receiver is disclosed in U.S. Pat. No. 4,370,753.
Referring to now to FIG. 3, a calling procedure of a conventional paging receiver operating in a battery saving function to receive a RF paging signal in a POCSAG code format is shown. Generally a preamble code (i.e., a first predetermined code) is searched for when the paging receiver is periodically turned on/off. If the preamble code is detected from a received RF paging signal, a word synchronization code (i.e., a second predetermined code) of a first batch is searched for a predetermined time. If the word synchronization code of the first batch is not detected within a predetermined time, the calling procedure returns to the search routine searching for the preamble code again. If, on the other hand, the word synchronization code is detected within a predetermined time, whether a self-address code (i.e., a third predetermined code) is transmitted or not, is found by checking self-frame data from the batch data, the calling procedure then returns to the search routine for searching for a word synchronization code of next batch data.
In more detail, under the operation of a battery saving function, the first predetermined code is searched for in step 11, and is monitored in step 12. If the first predetermined code is not detected, the calling procedure loops in step 12 until the first predetermined code is detected; and if the first predetermined code is detected, the call procedure advances to step 13 where a second predetermined code check counter X1 of the paging receiver is initialized to search for the second predetermined code for a predetermined time, while the battery saving function of the paging receiver is turned off; that is, the receiver circuitry 110 is continuously enabled. This is followed by step 14 where, whether or not the second predetermined code is detected is determined by checking the received batch data. If the second predetermined code is not detected in step 14, the calling procedure proceeds to step 19 to increase the second predetermined code check counter X1, and monitors whether an increased value of the second predetermined code check counter X1 exceeds a predetermined time in step 20. If the increased value exceeds the predetermined time, the calling procedure returns to step 11; and if it does not exceed the predetermined time, the calling procedure returns to step 14.
In step 14, if the second predetermined code is detected, the calling procedure advances to step 15 to initialize a code word counter X2 for checking a number of the received code words in one batch. Next, whether one code word of 32 bit unit has been received or not is checked in step 16. If one code word is received, the calling procedure proceeds to step 17 to search for the self-address code (i.e., the third predetermined code) among predetermined self-frame data and increases the code word counter X2. This is followed by step 18 where the increased value of the code word counter X2 is checked. In step 18, the calling procedure checks whether the value of the code word counter X2 is 16, since each batch of POCSAG data contains a word synchronization code (i.e., second predetermined code) followed by 16 code words. If the value of the code word counter X2 is 16, the calling procedure determines that it is time to receive the second predetermined code of a next batch and returns to step 13. Otherwise, the calling procedure proceeds to step 16.
In the returning process from step 18 to step 13, if an error which cannot be corrected occurs in the second predetermined code; that is, when the second predetermined code is missing from a RF paging signal, the program cannot proceed to step 14. Consequently, subsequent address code cannot be detected. Further, when the word synchronization code is missing, a check routine is continuously repeated for a predetermined time.
As described above, in the calling procedure of a conventional paging receiver, when the word synchronization code is missing, self-calling data cannot be received by the paging receiver.